Sanding disks

ABSTRACT

Accessories for an angle grinder include a disposable rotary sanding disk having quite large shaped ventilating/viewing apertures, for use with a resilient backing plate also having shaped ventilating apertures. The apertures of one or both parts are shaped so that snagging of the apertures on projections from the work surface is minimized and to facilitate air flow across the work surface during use. This air flow helps in cooling the work and ejecting detritus, so minimizing clogging effects. The ventilating apertures also facilitate viewing the work to be sanded through the spinning disk during the abrasion process, so that operator feedback is immediate. The holes also give the sanding disk more resilience so that a greater area comes in contact with the work and the disk wears more evenly over its abrasive surface.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of abrasive or sanding disks, and inparticular this invention relates to sanding disks and accessories forangle grinders and means for making them.

BACKGROUND

Abrasive disks, or sanding disks are widely used on portable electricdrills and (at a more professional level) on hand-held angle grinders.When used on these machines the disk is held by its centre against abacking pad and is rotated at generally a high speed while pressed infront of a backing plate against the work. The abrasive surface wearsdown the surface of the work by, in effect, a cutting action.Angle-grinder mounted sanding disks are commonly used (for example) inautomotive panel beating, where body filler is to be sanded back toconform to the original contours of a remodelled car part. It is saidthat millions of sanding disks suitable for use with angle grinders aresold each year. There are some problems related to the use of sandingdisks, such as:

(a) The relatively rigid backing disks commonly used with angle grindersanding disks force the sanding disks into an unsatisfactory mode ofoperation when the angle grinder is tilted towards the work duringuse--such as that primarily the edge engages with the work. resulting inlocal, intense action rather than an even, gradual action over a widerarea. There is a tendency for the work surface to develop anunsatisfactory scalloped surface which requires hand sanding blocktreatment. The disks cannot be used for finely controlled work such aspreparation of surfaces in a state ready for painting.

(b) Sometimes the material being abraded tends to melt at the highcutting speeds involved, and if this happens it is particularly likelyto clog the sanding disk in a quick and effective manner so that thedisk has to be discarded. Melting may also lead to the tool biting inand as a result the surface of the work may be inadvertently destroyed.Heating also adversely affects the life of the sanding disk.

(c) The operator cannot see the material being sanded during the actualoperation; he/she can only see material that is not covered by theblade. It is difficult to carry out a precise operation withoutrepeatedly inspecting the work in progress and more closely reaching anapproximation to the desired result. Hand-held tools cannot bere-applied precisely so that repeated inspection is not a good optionfor careful work.

It is a well known phenomenon that a disk having perforations becomessemi-transparent when spun at a moderate to high speed because of thepersistence of image on the retina in the human eye--the "persistence ofvision" effect. The image seen through a perforated spinning disk isfurther enhanced if there is a contrast in light and/or colour betweenthe spinning disk and its background and/or foreground. To increase thewidth of the "window" or see-through viewing effect when a disk is spun,perforations are usually designed to overlay each other. There are manyabrasive and rasping disks that make use of this phenomenon. Examplesare those of F. Reidenback filed Aug. 31, 1953 U.S. Pat. No. 2,749,681or J. C. Schwartz filed Mar. 26, 1985 U.S. Pat. No. 4,685,181.

Because of the presumed catastrophic consequences of protrusions intolarge apertures of perforated disks these inventions to date have reliedon using many small perforations in the disk in relation to total disksize.

DEFINITIONS & NOTES

Although we relate the invention to angle grinders in particular, theinvention is also applicable to sanding disks used in some other powertools, such as ordinary electric drills, even though the usual types ofelectric drills do not spin at such a high speed.

"Aperture" means a channel or hole passing completely through an object,and is surrounded on all sides by the material of the object. It is notlimited to apertures having a circular profile.

"Dished" means that a disk has been formed into a convex shape (like asaucer) and for this invention the abrasive would usually be found onthe base, or convex side, of the saucer.

"Disk" refers to a flat piece of relatively rigid material (thoughhaving some resilience) which is adapted for mounting on a rotatablespindle or arbor. It is not limited here to purely circular shapes. Itincludes materials adapted for use with an angle grinder in conjunctionwith a backing plate.

"Gap" means an indentation or invagination which is incompletelysurrounded by the material of the object. It would include thereforeconfigurations in which the circular periphery of a disk has had asegment, (defined below), removed or the configuration obtained by(notionally) moving an "aperture" until a portion extended beyond theperiphery of the disk.

"Sanding" is used herein to refer to any abrading or finishing operationin which the surface of a workpiece is treated to remove material oralter the roughness.

"Segment" means that portion of a circle which lies between theperimeter and a chord.

STATEMENT OF THE INVENTION

In a first broad aspect the invention comprises a sanding system for usewith an angle grinder or the like, comprising a disk bearing at leastone abrasive surface, the disk being adapted for mounting upon an arborof the angle grinder in conjunction with a matching backing plate,characterized in that the sanding disk is modified by being providedwith at least one non-concentric aperture adapted for viewing andventilation which aperture is capable in use of being substantially inalignment with at least one similarly adapted viewing and ventilationgap or aperture constructed within the backing plate, so that in use thework surface and the sanding disk are cooler as a result of airmovement, abraded material is moved tangentially away, and the user cansee the work through the at least one non-concentric apertures.

A more preferred number of non-concentric apertures is between three andfive.

Preferably the non-concentric apertures adapted for viewing andventilation are placed at varying distances from the centre of rotationof the sanding disk, so that when the disk is rotated, a substantialproportion of the area beneath the disk can be seen. Rotation of thedisk defines leading and trailing edges of the apertures and it is afeature of this invention that the trailing edge of each aperture isdisplaced out of the plane of the abrading surface of the disk andtowards the back of the disk. This has the effect of minimizing the riskthat protrusions from the surface being abraded will catch on the edgeof the disk and cause rupture of the disk.

In a subsidiary aspect the shaping comprises raking at least the leadingside, and optionally also the trailing side of the or eachnon-concentric aperture adapted for viewing and ventilation, therebyproviding at least one slanting side to the or each aperture. This isonly possible when the abrasive disks have significant thickness.

The distortion of the material surrounding the aperture so as to liftthe material away from the working surface on the intended trailingedge, may also be effective is causing air turbulence enhancing theremoval of swarf from the surface being abraded.

The invention also comprises a sanding disk as described previously, inwhich at least one edge of the or each non-concentric aperture adaptedfor viewing and ventilation is formed in order to serve as a cuttingedge.

In a further aspect the viewing or ventilation apertures may also beregarded as means to intermittently interrupt the abrading action of thedisk as it turns, thereby providing a "rest time" during which time thework surface may become cooler.

In another aspect the sanding disk as described previously may beprovided with one or more apertures primarily intended for alignmentwith alignment features upon the backing disk, so that the sanding diskcan on installation be aligned so that apertures within the sanding diskare matched with apertures within the backing disk.

Optionally the one or more alignment apertures may also serve asengagement means to mate with drive pins extending from the backingdisk.

Optionally, one or more apertures are provided in the sanding disk inpositions capable of matching air extraction apertures within a backingdisk.

In a preferred aspect the perimeter of the sanding disk may be distortedfrom a circular shape by the provision of one or more gaps, mostpreferably in the form of segments, around from the circumference of thedisk. Where a plurality of such gaps are provided it is preferred thatthey be symmetrically located to maintain balance in the disk.Preferably there are from three to eight gaps.

More preferably the number of gaps matches the number of non-concentricapertures adapted for viewing and ventilation. and are located on radiibetween those on which the apertures are located.

Preferably each gap has the shape of a straight line joining one part ofthe circumference to another. Otherwise expressed, the gap is formed byremoval of a segment of the disk.

Preferably the dimensions of the or each gap are adjusted so that whenthe sanding disk is rotated, it is possible to see through the disk inthe zone outside that of the viewing/ventilation apertures, and as faras the edge.

Optionally this type of gap may be used advantageously in the procedureof cutting sanding disks from stock material, by bringing disk centrescloser to each other and having common edges between adjacent disks, soas to minimise waste.

Optionally some or all gaps may have a curved outline.

A preferred curved outline is one that is drawn in towards the trailingedge of a viewing/ventilation aperture, thereby providing a narrowed orweakened zone capable of being torn should a projection engage with theviewing/ventilation aperture.

The surface of the abrasive disk can have a number of configurations. Ina first embodiment the surface is provided by a coating of abrasiveparticles adhered to the surface of the disk by a binder materialselected from cured resinous binders or metallic bonds. In a furtherembodiment the surface of the disk comprises a non-woven layer of fibershaving bonded to the fibers a plurality of abrasive particles. Suchnon-woven layers are conventionally bonded to a backing materialimparting a higher degree of dimensional stability to the whole diskstructure.

In still another aspect the sanding disk may be provided with one ormore peripheral folds--or "wing tips"--that are directed away from theabrasive surface, so that when the disk is rotated air is caused to movethereby further cooling the work area and directing the abraded materialaway.

In a related aspect a skirt may be provided around the guard of theangle grinder so as to confine the air brought into motion by the wingtips.

In yet another aspect the sanding disk is also provided with one or moreshearing sites, "tear zones" or deliberately provided points of weaknesscapable of disconnecting the disk from the drive means of the backingplate if the disk inadvertently engages with an object and attempts totransmit a high torque to the backing plate and to the angle grinder. Apreferred shearing site comprises a weakened zone concentric with themounting means or aperture.

Preferably this weakened zone is formed from a series of apertures cutinto or through the material of the sanding disk.

Optionally this weakened zone is formed from a series of slits cut intoor through the material of the sanding disk.

Preferably a disk retaining nut tightened onto the arbor of the anglegrinder is capable of retaining the tom-off sanding disk; preferably bymeans of a concentric, outwards-directed projection or the like providedtowards the periphery of the disk retaining nut, the projection having adiameter large enough to include the weakened zone.

In any case the sanding disk should preferably remain substantiallydynamically balanced about its axis of revolution.

Preferably the disk are used with a backing plate made of a resilientmaterial, and preferably the material of the backing plate has a darkcolour.

Preferably the backing plate includes at least one gap or aperture,positioned so as to be capable of alignment with the one or morenon-concentric apertures adapted for viewing and ventilation providedwithin the sanding disk.

Preferably the or each gap or aperture in the backing disk is similarlyprovided with slanted or raked surfaces, and optionally each aperturemay be provided with an air scoop.

Optionally the backing plate may be provided with further aperturessubstantially not capable of alignment with the non-concentric aperturesadapted for viewing and ventilation in the sanding disk and one or moreof the further apertures may be used for alignment purposes.

One or more of the further apertures may be used for purpose of drivingthe sanding disk, by means of engagement means held within said furtherapertures.

One or more of the further apertures may be used for air and materialremoval purposes; being connected to air extraction channels within thebacking plate.

Preferably such extraction channels run outward from the removalaperture towards the periphery of the backing plate, so that in use airis moved through the channel by a centripetal force.

Yet further apertures in the backing plate may be provided in order togive the backing plate a weakened zone that may be ruptured if aprotruding object is caught in a viewing/ventilation aperture.

Preferably the resilience of the combination of sanding disk and backingplate is sufficient to provide a significant flexibility of the activelyabrading disk during use, so that more than just the edge of the diskcan be in effective contact with a work surface.

In an alternative embodiment the backing plate itself is provided withclutch means capable of becoming disengaged from the drive shaft if thetorque applied through the clutch means exceeds a pre-set limit--as forexample if the backing plate inadvertently grips an object.

Another preferred embodiment of a clutch means is an overload clutchbuilt into the material of the backing plate. This may comprise a shearpin.

Yet another preferred embodiment of a clutch means comprises amodification by lengthening of the shaft of a retaining nut and amodification by provision of a shaft for a thrust washer so thattightening the retaining nut against the thrust washer (when mounting asanding disk and a backing disk forms an overload clutch acting in amanner analogous to a shear pin, allowing slippage. in the event ofexcess torque, between the backing plate and the retaining nut/backingwasher assembly.

Preferably at least one hole in the backing plate and at least one holein the sanding disk may be used in conjunction with a locating peg orpin to rotationally align the sanding disk on the backing plate so thatthe apertures are substantially in alignment. Preferably the locatingpeg or pin is removed after attachment of the sanding disk and beforeuse.

Optionally a locating pin or projection included in a sanding disk andfor alignment purposes inserted into the backing plate may also actduring use as a shear pin.

Optionally an overload clutch may include serrations or the like capableof creating a vibration or noise against a projection when the clutch isslipping.

Preferably the invention also provides a guard for an angle grinder,adapted to protect the user from injury resulting from the spinningsanding disk and/or the backing plate; the guard comprising a protectivecover mounted at least one of the threaded sockets for the grippinghandle and projecting forwards between the sanding disk and theoperator.

Preferably the guard is made of a tough clear plastics material;alternatively at least a part of it may be made of metal. Alsopreferably the guard is fixed in place. Alternatively however the guardmay be adjustable and moved forwards or backwards from time to time,thereby acting as a gauge plate.

In a further broad aspect the invention provides a process and apparatusfor the manufacture of preferred shapes of abrasive disk by using aliquid lance or liquid cutting process, in which a liquid emerging froma small nozzle under high pressure; the nozzle being capable of movementrelative to one or more layers of an abrasive sheet, cuts through theabrasive sheet to separate sanding disks and/or flaps.

Alternatively the cutting process may be a burning process using intenselight, as from a laser. Preferably the movements and cutting actions ofthe cutting process are controlled numerically from a stored sequence ofinstructions. Preferably the cutting process uses an array of nozzlesworking simultaneously in order to make a number of shapes at one time.

DRAWINGS

The following is a description of a preferred form of the invention,given by way of example only, with reference to the accompanyingdrawings in which:

FIG. 1: shows outlines (plan view) of a preferred three-hole abrasivedisk or sanding disk, according to the invention.

FIG. 2: shows outlines of a preferred five-hole abrasive disk or sandingdisk, according to the invention.

FIG. 3: shows outlines of three preferred backing plates, each havingthree viewing or ventilation gaps, according to the invention.

FIG. 4: shows two outlines of preferred backing plates, according to theinvention.

FIG. 5: shows the profile of a preferred aperture or gap in a sandingdisk or a backing plate, adapted to prevent against catching protrusionsfrom the work surface, according to the invention.

FIG. 6: shows the side view (elevation) of a preferred backing plate,according to the invention. One type of a locating pin and an aperturefor it in the backing plate are shown. This figure also includes asection through a backing plate having a raked hole and an air scoopaway from the abrasive surface, and a lifted trailing edge on theabrasive surface.

FIG. 7: shows the front and rear surfaces of another preferred backingplate, provided with cooling channels according to the invention.

FIG. 8: shows the side (elevation) view of a preferred abrasive disk orsanding disk mounted upon a backing plate and provided with studs forengaging with an abrasive disk.

FIG. 9: shows the user's view (elevation view) of a preferred abrasivedisk or sanding disk (of FIG. 1) mounted upon a backing plate (of FIG.4) according to the invention.

FIG. 10: shows a preferred abrasive disk or sanding disk provided withraised areas trailing the three large apertures, and a shearable or weaksection (three types of weakened portion are included in the onedrawing), and three versions of a holding nut for fixing it to an arborof an angle grinder.

FIG. 11: shows in section three versions of a backing plate providedwith clutches for slipping in the event of too much torque beingapplied.

FIG. 12: shows the working face of an abrasive disk or sanding diskprovided with multiple flaps of abrasive material according to theinvention. (Two flap orientations are shown in the one drawing).

FIG. 13: shows the working face of another abrasive disk or sanding diskprovided with multiple flaps of abrasive material according to theinvention.

FIG. 14: shows the working face of an abrasive disk or sanding diskprovided with multiple (10) holes, wherein the positioning of holesallows viewing through a substantial portion of a spinning disk.

FIG. 15: shows the working face of an abrasive disk or sanding disk of atype using a sandpaper manufactured with a contact adhesive surfaceaccording to the invention. (See FIG. 23 also).

FIG. 16: shows the rear (non-sanding) face of several versions of anabrasive disk or sanding disk of a type with one or more segmentsremoved, having increased edge visibility during use. The insets showhow such disks can be cut from a sheet of material with relativelylittle waste.

FIG. 17: shows the rear (non-sanding) face of a backing disk of a typewith one or more segments removed, having increased edge visibilityduring use. Extra raked cooling holes are also provided.

FIG. 18: shows a hole in a sanding disk or backing plate, with itsnon-catching capability enhanced by forming (as by pressing) a trailingedge deformation in the material, according to the invention.

FIG. 19: shows in section a further preferred clutch assembly for asanding disk for an angle grinder.

FIG. 20: shows some designs for a guard for an angle grinder to be usedwith sanding disks according to the invention.

FIG. 21: shows a way to cut multiple or single stock abrasive sheet witha high pressure jet of liquid to make sanding disks according to theinvention.

FIG. 22: shows some ways to pack cut-outs together in order to save onstock abrasive sheet.

FIG. 23: shows ways to lay and shape adhesive-backed sanding disks ontoa foam backing plate, the disk and the plate being modified according tothe invention.

FIG. 24: shows a sanding disk with (a) non-catching apertures and (b)alignment holes within a tear-out zone.

FIG. 25: shows a sanding disk in correct alignment on a backingplate--operator's view.

FIG. 26: shows a backing plate having a grip pad--like a ring ofsandpaper--intended to grip a sandpaper disk (such as FIG. 24) insideits tear-out hole zone.

FIG. 27: shows a backing plate suitable for use with a contact sandingdisk.

FIG. 28: shows one version of a contact sanding disk with (a)vision/cooling apertures, (b) indexing/alignment holes, "(c)" foldlines, and (d) vacuum apertures.

FIG. 29: shows another version of a contact sanding disk with (a)vision/cooling apertures, (b) indexing/alignment holes, c fold lines,and (d) vacuum apertures.

FIG. 30: shows a four-sided sandpaper disk with (a) wing tips, (b)air-scoop holes, and (c) a tear-out hole zone.

FIG. 31: shows the four-sided sandpaper disk in position upon a backingplate.

FIG. 32: shows a backing plate compatible with the sanding disk of FIG.30, having (a) a grip pad, (b) cooling channels, (c) a structurallyweakened breakout zone, and (d) index alignment means.

FIG. 33: shows a backing plate in section and a matching four-sidedsanding disk, having apertures, break-out zones, and a concentricweakened or tear-out zone. The backing plate has a grip pad--like a ringof sandpaper--intended to grip a sandpaper disk inside its tear-out holezone.

FIG. 34: shows a three-sided sandpaper disk in position upon a suitablebacking plate.

FIG. 35: shows a backing plate compatible with the sanding disk of FIG.36, having (a) a grip pad, (b) cooling channels, and (c) index alignmentmeans.

FIG. 36: shows a three-sided sandpaper disk with (a) wing tips, (b)apertures, and (c) a tear-out hole zone.

FIG. 37: shows a backing plate in section and a matching three-sidedsanding disk, having apertures, break-out zones, and a concentricweakened or tear-out zone. The backing plate has a grip pad--like a ringof sandpaper--intended to grip the sandpaper disk inside its tear-outhole zone.

FIGS. 38-41 are graphs and a bar chart showing comparative performanceof disks according to the invention and prior art disks.

PREFERRED EMBODIMENTS

The accessories to be described herein for use with an angle grinderinclude a disposable rotary sanding disk (where "disk" is as definedabove) having one or more relatively large viewing/ventilationapertures, and a resilient backing plate, also having similarviewing/ventilation apertures which has been developed particularly foruse in conjunction with the disk. The large apertures allow the operatorto see the work surface while it is being abraded. It appears that thelarge apertures are also of great benefit by allowing the work surfaceto stay significantly cooler than when a prior-art unperforated disk isused.

Fears as illustrated by what is available in the prior art--that theholes might entrap projections from the work surface--are unfounded intrials; the high rotation speed together with raised trailing edges onthe holes appears adequate to prevent a projection from entering theapertures of a spinning disk. The holes also assist in providing thedisk with more resilience than has usually been expected of a sandingdisk. Means (see FIG. 6 and FIG. 9 and particularly FIG. 23) formounting the disk on the backing plate in alignment may also beprovided.

Observations made by the use and developments of this invention haveestablished that a definite increase in efficiency and performance insanding disk operation is achieved by the creation of air turbulencebetween the spinning abrasive surface and the work surface or materialbeing abraded This appears to generate a significant cooling effect.There is also a benefit from intermittent cutting--allowing a smallmeasure of time to elapse between cutting intervals. There is a "resttime" occurring several times during each revolution of one of ourimproved sanding disks. It has been determined that the best results areachieved by using a small number of large perforations set back at anappropriate distance from the perimeter of the sanding disk and spacedat positions around the sanding disk, so that the balance of the disk isnot upset. We also provide optional gaps in the originally substantiallycircular periphery. Perforations are preferably raked to increase airflow in conjunction with the backing plate, with increased coolingbenefits also gained by incorporating extra ventilation between thebacking plate surface and the sanding disk. A by-product of this coolingmethod has proven to be excellent see-through capabilities whilst inoperation.

A quantitative scientific investigation of these effects would requiresophisticated equipment, such as a thermal camera looking through diskapertures to view and measure the temperature of the surface beingsanded (at a calibrated rate) by various disks under trial, or airflowmeasuring devices, and presumably there are standard test methods todetermine the lifetime of sanding disks when used in various ways.

The prior art in this field, being concerned about disk collapse andcatching protrusions, has relied on using many small perforations in thedisk in relation to total disk size. Our invention has also providedsafety tear out centers and release mechanisms built into the backingplate as well as the benefits of much increased cooling air flow.Resilience also reduces the suddenness of onset of abrasion against asolid surface. The indexing alignment features of this invention areusefull as is the option to increase unit production from the same givenamount of "raw" product.

In contrast to the prior art our invention uses a small number of largeventilation/viewing perforations in proportion to the sanding disk size,and with the exception of flapper disks, relies on a specialrelationship between a modified backing plate and modified fibre andfabric-based sanding disks. This invention also makes possible a moreflexible and controllable sanding operation not normally associated withangle grinder usage.

The sanding disk is preferably of the usual industry-standard diameter;usually between 4 and 7 inches (or a metric equivalent) and can be madeof the usual reinforced fibre base to which an abrasive surface has beenmade adherent. The material from which the disk is made can however alsobe plastic, such as a film, paper or even metal. Metal disks are in factpreferred where an abrasive, especially a superabrasive such as diamondor CBN, is metal-bonded to the surface of the disk to provide theabrasive surface.

The disk is typically used in conjunction with a backing plate where ithas insufficient strength to be used alone. This is indeed most oftenthe case since the disk is intended to be readily replaceable and usablesupported on a standard backing plate. It is however possible that thedisk is integral with its own backing plate which has the same overallshape as the disk and which confers the necessary rigidity anddimensional stability. Such a disk can then be attached directly to thearbor of a rotary grinder. This option is particularly preferred whenthe disk is already required to be dimensionally stable to perform inthe intended manner. Such disks are referred to herein as "rigid disks"to distinguish them from the disks primarily intended to be used inconjunction with a backing pad. Rigid disks include for example flapdisks, (as hereinafter described), disks in which the abrasive surfaceis provided by a non-woven fabric having abrasive particles adhered tothe fibers thereof (as hereinafter described), and metal disks bearingparticles of a superabrasive metal-bonded to a surface thereof. In suchcases it is preferred that the rigid disk has a recessed portionsurrounding the mounting aperture so that the disk can be used flatwithout the mechanism for attaching the rigid disk to the arbor of thegrinder coming into contact with the work surface. In such rigid disksthe integral backing plate has the same apertures and the same basicshape as the disk.

The disk has a central mounting or attachment aperture, and in additionhas a number of apertures which have the combined purposes of (a)providing a flow of air over the work surface, (b) allowing the operatorto see the work while actually abrading it and (c) making the diskbacking material less rigid, and alleviating possible stresses withinthe disk material. (Optionally a contact adhesive may be used to fix thedisk to a backing plate (see FIG. 15) or "Velcro"(™) or the like may beused). Prior-art apertured sanding disks are known (e.g. Bosch and seeabove) but those on sale are used solely as part of a dust-extractionsystem and the extraction system prevents viewing. The typicalappearance of prototype sanding disks is shown in FIGS. 1 and 2--wherethree holes in FIG. 1 are shown as 101 (the central mounting hole is102) and FIG. 2 illustrates that the invention 200 can have anyreasonable number of holes such as the five ventilation/viewingapertures here illustrated as 201, or the ten hole version of FIG. 14. Aone-hole disk (with a balancing segment removed from an edge) is shownin FIG. 22. The invention is of course not limited to the embodimentsillustrated. The example of FIG. 2 also includes an array of holes 203used as a deliberately weakened region (see later) and also non-circularapertures 202, which are substantially radially oriented slots.

Later in this Application we shall describe our optional vacuumapertures. They are placed close to the centre of our sanding disks andare aligned with apertures in the backing plate, similar to the Boschprior-art, except these apertures draw their vacuum not from the fanbuilt into the motor of the power tool or some other external source butfrom ducts sandwiched inside the backing plate or open channels, betweenthe backing plate and the sanding disk paper. The centripetal forcedeveloped on air occupying the ducts will, when the disk is spun, createthe required vacuum in the ducts. Dust can then be blown into acollection trap that then funnels dust into a collection bag. To helpthe process, the periphery of a backing plate can have veins or scallopsmoulded into its edge (circumference).

In one preferred form, the sanding disks are adapted to be used with aconventional angle grinder of the widely used type having a typicalno-load rotation speed of 11,000 rpm, driven usually by a universal(AC/DC) brush motor. Conventional angle grinders provide a drive shafton to which various disks (normally of abrasive material) may be mountedand spun at a high speed. A typical angle grinder is the single-speed115 mm grinder sold as the--"AEG WSL 115"(™)(600 watts). This size ofmotor provides an acceptable power for the prototype disks, whichgenerally draw less power than "solid" prior-art disks though having anequivalent performance. Here, it is thought that air-bearing effects,rest-time effects, and cooling may be responsible.

VIEWING

Apertures or perforations (101, 201) in the disk are provided in part sothat the user can see the material to be abraded through the spinningdisk as he/she is using the grinder, generally by drawing the tooltowards himself/herself. For convenience the apertures are circular orat least have no sharp or narrow comers because of the higher risk ofpropagation of cracks from stressed areas as opposed to circular holes.Nevertheless we show a diamond-shaped, raked hole in FIG. 2 as oneoptional shape. Holes having a narrow end and a wide end (perhaps thenarrow end is placed at the leading edge) can be used as one of manyoptions. Many other options exist; such as narrow slots running at anangle to radius lines or perhaps along curves that follow stress linesof the disk when in use. Three 22 mm diameter holes 101, equidistantfrom the centre have been used in early prototypes but many othercombinations are possible. Clearly, hole positions should preferably beselected so as to retain the balance of the cutter, and cutters may bebalanced dynamically by removing material from hole edges.

In relation to the viewing aspect, it is very useful to be able to seeand monitor the abrading action while it is in progress. Most sandingdisks do not allow viewing to occur during sanding. The anatomy of anangle grinder does allow viewing through the outer half of a spinningdisk, and these sanding disks have been developed to take advantage ofthat construction. If sanding is carried out with an opaque disk (theusual situation) the operator has to make a series of test abrasions,each time removing the tool to view the result, and as the job nearscompletion these inspection pauses have to be more and more frequent.The job completion process is a kind of successive approximation, andthere is a possibility that the abrading process will be taken too far.Using the present invention the operator can carry out an abrasionoperation in one application of the tool to the work and there is littleneed for judgement as-to the speed of wearing down, and the risk ofgoing too far. It is perhaps surprising that the presence of substantialapertures in the disk and the backing plate does not (as one mightexpect) allow protruding objects to entangle with the hole and causecatastrophic disruption to the sanding process. In fact one can bringthe spinning disk down hard onto a protruding nail and watch the nailbeing worn down with little or no problem, though for safety reasons onemight prefer to arrange that the disk meets the nail at an angle lessthat 90 degrees in order to reduce the risk of the nail digging into thedisk or the backing plate.

We have realised that designs having circular outer profiles have notaddressed the problem of concealment of portions of the work at theextreme edge of the rotating disk. Disks from FIGS. 1 to 15 havecircular profiles. Therefore we have invented a disk 1600 having severalsegments 1603 removed, as shown in FIG. 16. These segments may bestraight (1603), or curved (1604) or even gap-like (1605). There may befrom one segment upwards; while we prefer three or four in the prototypedisks, five (see 1605) or six are feasible and it would be possible(FIG. 22) to make a disk having an eccentric edge (one indentation orgap) balanced by one or more apertures elsewhere. As a result, the workbeneath the disk can be viewed right up to the edge of the disk, if theremoved segment in one place overlaps with a hole in another part of thedisk, and so the entire working portion of the disk "greys out" duringuse. (This lack of obviousness may lead to a hazard--see the section onguards later).

Disks in which the edges were scalloped or given a toothed appearancehave been used in the past. This was done primarily to make the edgesmore flexible but also to prevent or limit abrasion in tight corners.The edge treatments did not confer visibility of any part of thegrinding area because the disks were used with solid backing plates. Thelack of grinding performance at the edges was an intentionalcharacteristic of such disks and this clearly distinguishes them fromthe present invention. The disks were also not provided with aperturesin the body of the disk to permit viewing and/or cooling.

On advantage of removing chord segments from the disks is that, at thetime of stamping disks out from the original stock material, the centreof each disk may be brought slightly closer to adjoining disk centres,so that more disks can be cut one by one or in stacks (if the stock ismulti-layered) from a given area of stock material, as shown at 1606which is one example of closer packing of disks having segments cut off.This reduces manufacturing costs. Indeed, the inner profile of onesegment may comprise the circumference of a neighbouring disk. Thisinner profile may be a deeper indentation (called a "throat": more than5 throats may be a satisfactory number), or may be curved, with asharper leading angle and a shallower trailing angle. Possibly thestamped-out portions can be recycled and used on flap disks. FIG. 21shows an example flap at 2114 and how 15 flaps (2115) can be cut at thesame time as one disk is made, leaving very little waste material.

While it might be thought that removal of segments would result in ahigher risk of marking the work because of an irregular rim, theresilience of the rim that we seek in our versions together with highcutting speeds seems to minimise that risk.

AIR COOLING

There is a detectable current if not a blast of air emergingsemi-tangentially around a spinning disk made according to the inventionand rotated at the typical 8000-11000 revolutions per minute typical ofa 4.5 inch/115 mm angle grinder. It appears that the raked holes fromthe rear (the operator side) cause significant air turbulence at theabrasive surface and swarf tends to be expelled out to the sides orthrough the apertures. During use against a surface in somecircumstances, air may be carried to the surface presumably as shown inFIG. 6 and here it helps to cool the work, blow dust away from the siteof abrasion, and remove broken-off abrasive particles (which being hardare likely items to cause abrasion of the tool itself) from the workingare This is most likely to occur using the air scoop illustrated in FIG.6 and this is worth explaining. The arrow 615 shows the direction ofmovement of the backing plate in relation to the air and the worksurface. The portion of the backing plate leading the aperture 612 iscut away, and the trailing edge 613 may be brought upward as a kind ofscoop, so that some air is rammed into the aperture 612. There may wellbe significant compression as the air reaches the surface being abraded(at around 616) where we usually raise a portion of the backing plateand sanding disk trailing the aperture. (This raised portion also helpsto mimimise the risk of catching a protrusion). The air may also act asa kind of bearing, forcing itself between the spinning disk and thestationary work in a manner analogous to an air bearing. At the rear ofthe sanding disk, which tends to flex against the backing disk when itis pressed against the work there is also some to-and-fro air movementwhich will help to forcibly cool the back of the sanding disk. We alsoprovide slanted channels as an option--see the discussion of theembodiment described in FIG. 17. Normally however the contours of theback of the backing plate often generate a negative pressure within theaperture through the backing plate and this may give rise to an air flowwithin the aperture in the opposite direction, that is, away from thework surface. In either case there is turbulence generated at the worksurface and this helps significantly in swarf removal. Carefulcontouring of the aperture openings in the backing plate can enhancethis effect.

While a rake (or slant) of the leading and trailing edges of the holesthat are made through the sanding disk itself might, in addition toproviding snagging protection, somewhat enhance air flow, it isgenerally difficult to produce a substantial air turbulence effect insuch a thin material and this function is preferably provided largely bybuilding a rake effect into the backing plate, which may be 3-5 mm thickin the region of the holes. This is shown in FIG. 6; a shaped sheet isshown in FIG. 5 or FIG. 18. (Of course a thicker sanding disk will becapable of supporting fully functional raked holes and could show theclaimed effect even in the absence of a backing disk. Commercially, mostabrasive material is sold as thin sheets for use with a backing plate.).Consequently the leading border of each hole is slanted away from theperpendicular. FIG. 5 shows the preferred arrangement and in thatdrawing 500 is a cross section through a portion of a sanding disk orthrough a backing plate, including a gap or aperture. The preferreddirection of rotation is indicated by the arrow 507 and the abrasivesurface is downwards. The leading edge 505 of an aperture or gap 502 isslanted to leave an acute angle at the edge closest to the abrasivesurface, while the trailing edge 504 is slanted so that an obtuse angleis closest. (506 shows a further raking shape which may be used tominimise the risk of the disk catching a projection). Even without anactual raking of the sanding disk apertures themselves, there issignificant and useful air turbulence caused by the motion of theapertures in the backing plate when the disk spins at a high speed. Wecannot measure the actual air movement with the equipment we have atpresent. All that we can determine is that the work surface stayssignificantly cooler.

We have developed a preferred way to provide a raked hole effect in anordinary sanding disk of a typical thin material. This comprises apressing operation that deforms the material of the disk so that theportion of the disk immediately trailing the hole (when rotating in itspreferred direction of rotation) is pushed away from the abrasivesurface. FIG. 18: shows a raked hole 1801 within a sanding disk 1800,its capability enhanced by forming of the material of the sanding diskor backing plate, according to the invention. The leading edge 1803 isgenerally not deformed but the trailing edge 1802 is bent away from thework surface. The region 1804, though abrasive, is unlikely to catch ona projection even if the disk is turning slowly because it is at agentle slant. By incorporating such a deformation, the principles of theinvention can be applied to a disk alone, without requiring a backingplate having raked holes. The forming process can be a simple pressingoperation carried out between suitable dies at the time of stamping ofthe sanding disk from bulk sheet abrasive material.

Even though we have observed that there is little likeliness of catchinga projecting object at the trailing edge of a hole, or the like, (partlybecause there is a new hole presented during use (10,000 rpm) at aboutevery 2 mS) the deformation shown in FIG. 18 helps to minimise the risk(such as when the tool is slowing down) by providing a gentle slope forthe object to glance off, rather than an abrupt comer to engage with it.

The air movement has a cooling effect. We have observed the temperaturereached by an iron object (a nail) while it is being abraded by thesanding disk. (Nails are a useful test object because they are oftenencountered during sanding operations on used wood). When using aconventional (entire) sanding disk the head of the nail may becomered-hot and will certainly burn a finger. A conventional sanding diskwill be destroyed by the heat. When using a perforated sanding diskaccording to the invention, the nail, though being worn down at acomparable rate, remains cool enough to be touched. The adjoining timberis not overheated and burnt or at least discoloured. One test reportedan about 120 deg F. reduction in temperature over that produced by useof a plain sanding disk, but the exact operating parameters are notknown.

Two backing plate or disk outlines are shown 300 and 400 respectively inFIGS. 3 and 4; FIG. 4 is "improved" in that the periphery of the disk isextended outwards from the position (shown by dotted lines 301) of FIG.3. These backing disks include gaps 303. The arrow 403 shows thedirection of rotation. It is possible to produce a resilient backingdisk that extends to substantially the full diameter of a sanding diskand in this case it may be preferable to provide apertures rather thangaps. Preferably the number and placing of holes in the sanding diskmatch those of the backing disk. In use, the operator placing a sandingdisk on a grinder might visually align the ventilation/viewing holes 101in the sanding disk with the gaps or holes 303 in the backing disk. Orhe/she might use a locating peg or pin (that shown at 603 in FIG. 6 isone embodiment; FIG. 23 is another) in order to hold the disk in placeduring rotation of the tightening nut. This is a relatively precise wayto align the disk. Preferably the locator peg is removed before use.FIG. 9 shows at 900 a sanding disk 100 beneath a backing disk 401, withthe holes of the sanding disk in good alignment with the gaps of thebacking disk. FIG. 9 also illustrates a sanding disk having locatorholes 905 which substantially match holes 601 in the correspondingbacking disk.

Interestingly, the backing disks of this invention assist ordinarysanding disks--those that are solid disks--thanks to their resilience.

FIGS. 6, 7, and 8 show some preferred backing plates from theside--elevation view. That of FIG. 6 (600) is preferably made of aresilient compound such as a rubber or a plastics material and isrelatively stiff because its profile remains thick relatively close tothe edge. Note the locator hole 601 for use with a locator peg 603. Thebacking plate of FIG. 8 (at 800) is more resilient (assuming similarmaterials) because the outer portion is relatively thin close to theedge. FIG. 8 also shows a curved or dished shape which we have foundpreferable--it allows use of the resilience of the sanding disk itself(803 in FIG. 8) alone when lightly sanding an object. A flat sandingdisk may, after some use itself may take on a slightly dished appearancebecause of the way that force is applied about the edge of the disk.Perforated disks are more resilient than unperforated disks.

FIG. 6 also includes one means (of many possible methods) toconveniently set the orientation of the sanding disk in relation to thebacking plate, when mounting a new disk on an angle grinder. There is aset of holes 601 provided in the backing disk. Corresponding orientationholes 905 are provided in sanding disks, and as can be seen, these arepreferably in a fixed relationship to the repeating structures of thesanding disk, so that for example three possible satisfactoryorientations of the sanding disk results in three holes 905. Whilemounting a sanding disk and before the retaining nut is tightened, theoperator pushes a locating peg or pin (shaft 603 and head 604) throughthe disk and into the corresponding hole in the backing plate so thatthe disk is held in substantially the correct orientation whiletightening the retaining nut. The locating pin, which may be made of aplastics material, is then removed. In practice a typical operator mayuse a nail or the like as a substitute for a locating pin, and clearlyit is useful to remove the nail before commencing use. (Locating pinsmay be cheap enough to pack with every sanding disk). It may bepreferable to make sanding disks with locator peg structures permanentlyattached to the rear of the disk, although at the present time disks aresimply stamped out from stock sandpaper sheets. In that case the locatorpeg structures may serve a dual purpose of shearing and giving way iftoo much torque exists between the sheet at the disk--if, for example, aprotruding object is inadvertently gripped.

We believe that many synthetic materials which are otherwise prone tomelt and then fill the spaces between the abrasive particles on asanding disk remain cooler and are less likely to clog and spoil thedisks of the invention. The disk itself presumably enjoys a longer lifeif it does not overheat.

Accordingly, we have added further holes in a backing plate. These maybe raked. Raked holes move air directionally, but even unraked holesimprove cooling. When the disk and backing plate are rotated, access isprovided for air to reach the rear of the sanding disk, and cool it.Raked holes increase the total flow of air and render it moreunidirectional, so are preferred though not essential. FIG. 17 shows therear (non-sanding) face of a backing disk 1700 of a type with one ormore segments 1701 removed, having increased edge visibility during use.Extra raked cooling holes 1702 are also provided. The segments 1701which, like the larger viewing apertures, are intended to line up withcorresponding voids in the sanding disk in order to provide visibilityof the work during the actual sanding operation.

DISK PROPERTIES

The holes together with the preferred type of backing plate give thesanding disk more resilience than an ordinary disk used with an ordinaryhard backing plate. The normal pattern of use is to apply the spinningdisk to the work at a region near one edge and with the preferred degreeof resilience this may mean that the outer 1/3 to 1/2 of the diskmomentarily contacts the work during each revolution. Benefits of thisinclude that the disk wears more evenly over its abrasive surface.Examination of well-used disks show that the outer half (measured alonga radius) of the disk is relatively evenly worn, while portions near thecentral mounting hole remain largely unworn. The outer perimeter of thesanding disk is still present. (In contrast, an ordinary disk used withan ordinary hard backing plate tends to wear in a narrow perimetric rimand the material of the rim of the sanding disk is lost). We expect theaverage lifetime of a sanding disk to be increased by up to about 20%,even though there is less abrasive material included per disk.

We believe that the holes may take out some of the stresses that buildup in a sanding disk. It is common for a new sanding disk to be curledup when it is first taken from a packet. Attempts to straighten the diskcan lead to cracking of its adherent abrasive layer. Use of it in acurled state results in hard-to-control thumping. We have noticed thatdisks including holes are less likely to exhibit and hold the curlingphenomenon and show the consequential thumping effect when used.

Furthermore, the presence of holes makes the perimeter of a sanding diskaccording to the invention more flexible. This is quite useful for moregently abrading a surface. We have also taken advantage of thisflexibility by using a backing plate that has a smaller diameter thanthat of the sanding disk. A typical relationship is shown in FIG. 9where it can be seen that the backing plate reaches out to about thefurthest extent of the viewing/ventilating apertures. Although prototypebacking plates have a circular circumference, it may be preferable toshape the perimeter as in FIG. 4 in order to optimise the kind ofsupport provided to the sanding disk. Furthermore one preferred shape ofbacking plate itself has a slight cupping (see FIG. 8); that is, itsoutermost portions are slightly raised (taking a work surface as areference plane) as compared to the more central portions. This meansthat the backing plate provides very little support until at least somepressure has been exerted upon the disk. On the other hand, some flatbacking plates can provide a similar effect.

The disk/plate movement can assist air to reach the rear of the disk andcool it. We have also designed a backing disk having channels tocirculate the air in the space between the backing plate and sandingdisk. FIG. 7 shows the principles. The disk 700 shows the rear (operatorside) of a disk, with air holes shown at 703 and 705. Buried channelsspiral out through the substance of the disk to reach the sanding side(see 701) where they may lead into the viewing/cooling apertures 702 orbe made into channels 706 that lead out to the circumference.Centrifugal air movement occurs when the assembly rotates. This type ofconfiguration is useful with thick backing disks--such as the foam onesfavoured by auto refinishers.

Note that we have chosen to use a disk having a small number of largeholes primarily for viewing and ventilating purposes. (The word "hole"here means an aperture of any shape). It is possible to produce diskshaving many holes, perhaps even a hundred or so, if cooling and/orflexibility is the primary desired result. Nevertheless we mainly preferto develop the viewing/ventilating attributes, although there may besanding applications that we have not considered wherein resilience isof much greater importance.

Clearly the type of material used as a substrate for the sanding disk isof greater importance than may have hitherto been thought, particularlybecause the invention enhances the sanding process using an anglegrinder and a sanding disk, and makes it a more versatile and preciseoperation than has generally been believed. WE have concentrated on theanisotropic fibre backed disks rather than the type in which a textilehaving clearly oriented fibres is used. Centrifugal force tends torender a spinning disk less resilient--at least in the position where itengages with the work--than a stationary disk, but the principlesexplained herein still apply at normal angle grinder rates of rotation.

Backing plates are preferably coloured black, in order to enhance visualcontrast for a person looking through a spinning disk and relying onpersistence of vision to see the work behind. This colour is lessobtrusive than white, which tends to result in a graying out of a viewof a work surface seen through a white or other light-coloured disk.

BUILT-IN SHEARING

It is useful for the invention to include safety features so that if thesanding disk somehow tightly grips a workpiece during a sandingoperation it can be torn off the backing disk--or somehow disengagesitself from the driving system so that no further adverse consequencesfollow. FIG. 10 shows some variations by means of which the sanding diskitself 1000 can be made frangible. It is provided with shearing/tearingpoints 1003 (sharp-cornered apertures) or alternatively circularapertures at 1004, or alternatively a series of tabs 1006 directedtowards the centre so that the weakened zone gives way if an excessivetorque is applied. Other ways to impose a weakened zone can be used suchas 1010, 1003 and 1004, and a series of slits (which may or may notcompletely penetrate the material of the sanding disk) forming aninterrupted circular line 1008 is a further way to do that. A retainingnut 1001 for holding the sanding disk and the backing disk onto an arborof an angle grinder is also drawn; its sectional view is at 1005.Preferably the disk 1000 remains captive beneath the periphery of thehead of the nut after shearing, preferably provided with a raisedportion 1002 to allow slippage, so that the disk does not fly free ofthe tool and possibly cause injury. Most nuts have a chamfer 1007, asshown in the example 1006, to aid in gripping the disk. The nut of1011-1012 is designed to hold only the backing plate to the arbor, andassumes that the sanding disk is held onto the backing plate by othermeans, such as the projections 805 shown in FIG. 8. The disk in FIG. 10shows raised portions trailing the holes, as at 1013.

It is also possible to equip the backing plate itself with a clutch orreleasing type (shear pin) mechanism of some type so that excessivetorque cannot be transmitted past the clutch. Where plates having someform of gripping means over their entire surface are used, a clutchwithin the backing plate is preferable. This has the advantage thatsanding disks are not so often wasted, and it also provides for thesituation wherein some object engages with the backing plate itself,perhaps through the ventilation/viewing holes. (This is possible if avariable-speed angle grinder is driven only slowly, or if any anglegrinder is put down before it has come to a full stop and thestill-spinning disk engages with some generally protruding object). FIG.11 shows three examples in section; all of which can be made in aresilient material as a casting or forming operation. Feature 1102illustrates a V-shaped tongue-and-groove formation while 1104 shows amore tongue-like variant and 1103 shows a slip ring (which may beembedded in either the inner or outer portion of the plate, or evenboth. The version shown at 1102 may be liable to give way if too great aside force is applied. Any of these clutches may be provided with aregular distortion of the sliding surfaces (such as a ratchet type ofshape, or a shear pin 1106) so that slipping of the clutch is clearlyevident during use as a kind of vibration, noise, chatter, or freespinning and the operator will know to reduce the pressure applied.Holes to engage with a tightening spanner may be provided as at 1107.

An improved clutch or release mechanism for a backing plate for an anglegrinder can be made from a modified retaining nut and thrust washer, asshown in FIG. 19 which shows this assembly 1900 in section. The thrustwasher 1904 differs from the type normally sold with backing plates by(a) having the spigots (that engage with depressions in the backingplate) deleted, and by having an extended shaft. This and the extendedshaft of the retaining nut 1901 are made to be of such a length that,when screwed together by tightening the retaining nut about the backingplate 1907, the backing plate is gripped only tightly enough to hold itduring normal working torque. When excess torque is applied, the backingplate can slow or stop while the nut/washer assembly 1901+1904 continuesto be driven. Preferably there is some means to make a noise or causevibration so that the operator is aware that slippage is occurringbefore friction-developed heat affects the equipment. This may comprisea toothed hub 1909 in the backing plate, which engages with a pawl 1905,or a spring and ball, or shear pin, or the like projection(s) from oneor other of the thrust washer 1904 or the retaining nut 1901.(Alternatively the teeth may be included in the nut/washer assembly andthe projection in the backing plate). Possibly the combination of teethand pawl may themselves partially or completely define the torque atwhich the clutch gives way.

FIG. 12 illustrates a version 1200 of the sanding disk of thisinvention, bearing multiple flaps of abrasive material. These devicesgenerally come with their own backing plate 1202. Flaps may be attachedin radial lines as at 1201, or at a slant (as beside the marker 1202). Aseries of small holes 1203 provide a weakened zone in case the diskgrips an object, but a preferred weak point is a slip ring 1303 and ashear pin 1304. The tangential flaps-may tend to cause the wheel tobecome less dished when it spins.

FIG. 13 shows another (1300) sanding disk having flaps, where the flapsof abrasive material 1301 are interrupted by the apertures 1302. Thisgives the work surface a series of rest times and assists in cooling.FIG. 14 is provided to show that holes may be placed at variousdistances from the centre of the flapper disk, and preferably they arearranged so that the innermost perimeter of an outer hole 1401 is closerto the centre than the outermost perimeter of an inner hole 1402, sothat an operator can see through substantially all of the disk whenusing the tool. The holes 1403 (though not essential) are here providedfor imposing a weakened zone. Generally though the flaps will be tornoff if overstressed. Alternatively or additionally a clutch or shear pinarrangement or the like can be provided (FIG. 13). Similar holes couldbe used in the contact-adherent system of FIG. 15, where a sticky (or"Velcro" fitted) disk 1501 is stuck down over its entire surface onto adisk 1502.

MOUNTING THE DISK ON THE BACKING PLATE

Backing plates can be provided with a built-in thread matched to that ofthe arbor of the angle grinder. In that case they can also be providedwith holes to engage with a tightening spanner. Backing plates can beprovided with perhaps 3 to 7 stubby projecting pins that engage withalignment apertures stamped through sanding disks. Examples are shown inFIG. 8 which shows a backing plate seen from the side, with projections805 aligned with similar-sized apertures 806 in a sanding disk 803.(FIG. 23 shows another system). This avoids the need for a separate,fittable and then removable locating pin like 603 (which may becomelost), and the stubby pins, which are not long enough to reach the worksurface during use, also serve to lock the disk to the spinning packingplate during use. They transfer the torque from the arbor, via thebacking plate, to the disk. In the event of excessive torque, the stubbyprojecting pins may break off, or the sandpaper, otherwise only retainedon the arbor but not otherwise locked in rotation to it, may come out ofalignment with the stubby projecting pins.

Where backing plates include gaps to overlay sanding disk apertures,they can be made with gradual trailing edges so that if a projectiongets through a sanding disk it can tear out the edge of the disk andescape from the backing plate, probably causing a jerk to the anglegrinder but at least not continuing to be trapped. FIG. 9 shows this,along with a raked edge 904.

RESILIENT BACKING PLATES for FINISHING WORK

One preferred type of backing plate comprises a thick, foam-filled (sothat it is soft and resilient) backing plate, typically 24 mm thick and200 mm in diameter. This is used in conjunction with adhesive-backeddisks of sandpaper, and the combination is widely available andgenerally used for automotive finishing work. We modify the backingplate according to the theme of the invention so that it is fitted witha number of apertures--for (in combination) cooling and viewingpurposes, or just for cooling purposes, and we cut channels orindentations in the surface of the backing plate so that the risk of aprotruding object gripping the trailing edge of an aperture in aspinning disk is minimised. FIG. 7 shows one system for coolingchannels. FIG. 22 shows relevant diagrams; a fitting plate 2301, atypical pre-cut sanding disk 2320, and the front surface of the backingplate 2310.

A fitting plate for use with our modified foamy backing plate includesone or more locating pins 2302 placed so as to mate, when in the correctorientation, with locating holes 2312 constructed within the foamybacking plate 2310 and to be fed through holes 2322 in the sanding disk,which is placed, abrasive side down, upon the jig or fitting plate 2301prior to the above mating of locating pins with holes. Optionally,retaining clips may be used on the jig in order to hold flat any sheetswhich may tend to curl. When locating a sanding disk that can have (orpreferably has) only one orientation to the backing plate, it ispreferable that one locating pin is longer and preferably thicker thanthe rest. There are also preferred trough-forming projections 2302located upon the fitting plate 2301 at positions corresponding to thetrailing edges of the larger viewing/cooling apertures in the disk 2321and the backing plate 2311 (these holes preferably being raked as shownat 2316 and 2336). The projections push the covering parts of thesanding disk into recesses provided in the backing plate. (The diskpreferably has slits 2323 cut on the trailing side of the largerapertures to allow for this distortion). Once the backing disk islocated on the locating pins the disk can be pressed down against theadhesive surface and the viewing/cooling apertures will be placed insubstantially correct alignment. The fitting plate is then pulled off.As a result of the deformation of the sanding disk at the sites of theprojections 2303, the sanding disk is provided with pressed-in abrasivematerial on the raised-from-the work trailing edge of the largerapertures, to assist in minimising the risk of catching a protrudingobject during use. In addition air flow over the work originating fromturbulence caused by the viewing/cooling apertures assists in keepingthe cutting cool.

Further to this, we also provide a striker plate or attachable fittingsthat retain the sandpaper in position inside the troughs 2313 bygripping the bent-inward portions of the (usually) adhesive disk betweenthe fitting and the backing plate. These fittings 2334 may simply clipinto place using inherent shape and resilience, or they may be held inplace with fasteners, such as screws 2331. The fittings may also includeprojections 2332 which rise above the surface of the foamy backing plate2330 on the operator's side and act during use may act to enhanceairflow down the apertures and towards the work surface. Hence theabrasive surface 2333 is cooled, while the operator has some chance tosee the work through the same holes. (These air scoop formations areconcealed from the operator by remaining beneath the guard of the anglegrinder).

GUARDS

There is a small risk that the sanding disk of this invention, beingless concealed by a backing plate, may inadvertently cause deeperinjuries than prior-art sanding disks if inadvertently brought intocontact with a person. Therefore we have given consideration to guards,and FIG. 20 shows some designs. A preferred guard 2003 is mounted on theangle grinder body 2001, and comes forward over the sanding disk 2004 asfar as is necessary to provide protection. A preferred mounting siteemploys the threaded holes provided for the handles 2002, for these tendto be standard features between different types of angle grinder.Generally holes are provided on each side (as shown) but the operatorhas only one handle to be put in one side or the other depending onhandedness. The guard 2003 may be held between a handle and the body ofthe grinder, or it may be held in an un-used hole by a bolt. (The handlemay be placed on the right or the left side according to the handednessof the operator). A guard may be made by pressing or forming so thatlugs 2005 are bent upwards from the plane of the guard. A side view oftwo versions is shown at 2014; the lower one has at 2006 a slotted holeso that it can be moved forwards or backwards. Preferred guards aretransparent, so that the operator can see through them and may be ableto have the entire disk covered by the guard--yet still be able to seethrough the equipment to the work during abrasion. Another version isshown at 2015; this version is adjustable by means of a slot 2011, awaving nut 2012, and a pivot nut 2010, which allow the curved portion2007 of the guard to move forwards and backwards relative to the anglegrinder, onto which the guard is held by bolts 2008 and 2009 onto thebrackets 2013 entering the handle mounting holes. (The handle mayreplace one of the bolts). 2016 is an optional trough on the other side,to allow more flexibility in adjustment.

Preferred guards are also capable of adjustment to and from the edge ofthe sanding disk, so that the amount of exposed disk can be optimisedaccording to various working conditions.

In addition to the obvious safety considerations in favor of theprovision of guards, there is an added advantage in that anappropriately shaped guard will help channel air flow generated duringgrinding and ensure that swarf produced is ejected with the radiallyoutwardly, even when the air turbulence generated by the viewingapertures, especially as sculpted in accordance with a preferred featureof the invention, tends to draw air from the grinding surface backtowards the operator. Any such material is swept away by the swirlingair currents generated between the rotating disk/backing plate and theguard itself.

PREPARING DISKS FROM SHEET MATERIAL

Conventional disks, and particularly the sanding disks of thisinvention, are generally stamped out from stock sandpaper, generallycomprising fabric or fibre-reinforced backing material onto which theabrasive grains have been attached by a suitable type of glue, suppliedin rolls about 1.5 meters wide. The stamping act is carried out betweendies in a press. Naturally there is a significant amount of wear on adie working with hard abrasive materials, and it is expensive to makeeven a simple circular cutting shape, let alone the more complex shapesof the invention. Assuming NZD $20,000 for a die suitable for thisabrasive application, and a lifetime before iextensive repair of 150,000presses, one can see that the stamping cost per disk may be of the orderof 5c plus wages for the workers attending the machine and possibly theexpense of upgrading to heavier presses.

Accordingly we propose to use, at least for trial runs, a liquid cuttingprocess as shown in FIG. 21, in which a fine jet of water (or some othersuitable liquid) forced out of a nozzle at a high pressure is used tomake precise cuts in a sheet of stock sandpaper in order to preparesanding disks. (We understand that certain liquids are more beneficialto standard sandpaper stock; these may be used as the cutting fluid. Inaddition, abrasive granules may be added to the water stream as ispractised in the art (but see below). In more detail, the liquid cutterwould, as is customary in water cutting techniques used in otherfabrication processes, use liquid raised (in the supply pump 2103) to apressure of perhaps some 30,000 pounds per square inch pressure, broughtby means of a flexible hose 2104 to ultimately emerge from a nozzle 2105close to the material to be cut. There is preferably some means ofcontrolling the flow, such as a pressure relief valve or a bypass valve,so that the nozzles can traverse the stock material without cutting (asin order to reach a hole position). Spray and waste is collected,preferably actively with the aid of airjets and vacuum cleaners (notshown), and the fluid may be filtered well and re-used. The nozzle ismoved relative to the stock by computer control, preferably to aprecision of ±0.1 mm over the width of a single sanding disk, although aprecision of ±1 mm might be sufficient.

In one embodiment the sheet of stock coming off a roll 2101 may be movedforward and backward by gripping rollers 2109, one steel and one(against the abrasive side) of rubber, to cause movement in oneorthogonal axis, and the nozzle or nozzle array 2105 may be moved fromside to side on a rail or some other suitable support, in the otherorthogonal axis. Stepping motors (2106, 2107) coupled to rollers 2109,2108 represent one preferred source of motive power since they areeasily coupled to a computer-based controller 2110 by known interfaces.The HPGL plotter language (or similar) might be selected as astandardised way of instructing the stepping motor interfaces.Preferably the unit step size of the stepping motors in both axes issimilarly related to relative work/cutter movement so that when a circleis intended, it is obtained. (Software can compensate for constanterrors of scale, so the above requirement is simply a preferredfeature). Preferably a number of nozzles 2105 are held in a gangformation on a rigid beam or on a rigid plate 2113, so that a number ofidentical disks 2102 can be cut from the stock roll in one set ofcontrolled movements. FIG. 21 does not show the details of a practicalmachine. For example, the lengthwise movement of the stock shouldpreferably involve a low-resistance, low-momentum action and (as inreel-to-reel tape drives for computers) a loop of material may be drawnoff and reduced or lengthened as forwards or backwards movement occurs.In FIG. 21, the roller 2118 could be relatively lightly spring-loaded sothat it tends to push up. Motors such as 2117 driving the rolls areuseful to reduce drag on the rollers 2109 at the cutting machine.

The addition of abrasive to the liquid jet may not be necessary if themachine is made so that the jet first hits the abrasive side--for thenthat abrasive acts as the cutting abrasive.

It may be possible to prepare a stack of sanding disks 211 1 in one passfrom a multiply stock sheet. The effectiveness of this may be highlydependent on the coarseness of the grit and the thickness of the backingmaterial being cut. That is, too many layers will exceed the capacity ofthe cutting jet to make clean cuts. FIG. 21 shows an additional roll2116 behind a first roll 2101 and possibly further rolls of stock can beadded. Or the stock may be wound as a multi-ply single roll.

Of course, laser cutting may be used as an alternative (wherein aninfra-red transmitting lens for focusing radiation to a point; the lensbeing coupled to a carbon-dioxide continuous wave laser, replaces theliquid nozzle, but we understand that this is more expensive and takesmore skill to use and maintain the laser(s), and there will be noxiousfumes to dispose of, arising from the backing material and glues.

Sanding disks tend to curl up when packed and they are prone todeterioration if water gets into the backing material, particularlyduring storage. It tends to do this from cut edges. (This is a possibledisadvantage of water as a cutting liquid. Therefore, the cutting liquidmay also be provided with sealant properties. It may be a meltablesolid, such as a wax--that is molten when it is used as a jet. Some thatsets over the sanding disk, where it can then can act as a lubricantduring use. Or it may be water or a watery liquid including somedissolved material that acts as a varnish, or as a sealant. Or it may bea polymerizable material such as a polyurethane paint.

The advantages of CNC (computer numerical control)-based liquid cuttinginclude that it is now trivial to prepare and manufacture a new designof sanding disk of virtually any shape (2112 represents a set of cuttingco-ordinates), without the substantial expense of fabricating a veryhard die, wear is substantially limited to (replaceable andmass-produced generic) liquid nozzles rather than to re-sharpening andre-surfacing entire pattern-specific dies, and there is a possibility ofthe cutting sequence first preparing useable and recoverable flap shapes(style:2114) from within areas destined to become waste, and thencutting out the disks. Perhaps a retractable arm can catch the flaps andlift them from the cutting area. The illustration shows 15 flaps at 2115made from the otherwise waste stock around a single example aperturedand gapped sanding disk. Most sanding disk shapes occur in the librariesof typical computer drawing packages. Of course economy in cuttingstrokes leads one to prefer those shapes of sanding disk that includestraight (or other) edges common to more than one disk, as shown in theexample set 2112 which would result in very little waste, especially ifflaps 2115 are cut from the inter-disk diamond shapes and from thelarger disk apertures also.

The path of the cutters may be programmed so that all removed materialis shredded finely. When gathered up and filtered, this material can beused in the manufacture of grinding wheels of various types. In any casethere will always be some finely divided material recoverable from thefluid drains of the cutting machine.

Fluid cutting is less likely than pressing to initiate stresses at thetime of manufacture at a sharp corner or blind end of any cut other thana circular outline. (Cracks are expected to tend to propagate fromstresses arising at corners).

The preferred anti-snagging shapes to be provided about the trailingedges of the apertures cut through our type of sanding disk by creatinga raised "hood" over each hole are preferably created in a separatepressing step to the cutting step, whether the cutting step uses dies orotherwise.

It should be emphasised that the fluid cutting method of preparingsanding disks is also applicable to conventional sanding disks, that is,circular shapes with perhaps a central, concentric mounting hole and noother.

FIG. 22 shows some other possible layouts for sanding disks though it isimpossible to show all options. Presumably optimisation can be variedaccording to relative costs.

FIG. 22 shows, at 2202 a single aperture disk, having a balancingsegment removed from its periphery, and a mirror image at 2203.

The sanding disk 2400 of FIG. 24 has (a) three viewing and principallyanti-snagging apertures 2403 (which have been drawn to show the limitsof the preferred recess made by pressing the material of the diskinward, and (b) three drive/alignment holes 2401, at about the sameradius as a tear-out zone 2402. Preferably, all three of thedrive/alignment holes are driven by means of corresponding pins held inthe backing plate. The sanding disk, when connected to the drive pins,is in correct alignment on the backing plate. If the disk is, in use,exposed to too great a stress the drive pins will destroy the tear-outzone 2402, so that the disk will come free of the backing plate and thedisk can no longer be driven.

In FIG. 25, 2500 is the assembly, 2501 is a central register plate onthe backing plate, 2502 is the sanding disk, 2503 is a breakout zone onthe sanding disk, and 2504 is a sanding disk to backing plate alignmentaperture and/or pin. An advantage of this arrangement is that theprocedure for putting a disk on the backing plate is simpler and easier.

An additional enhancement to the backing plates of this invention is toprovide a grip pad 2602 for gripping the sanding disk by means of a nutpressing the disk between itself and the grip pad, inside the concentrictear-out zone. The grip pad 2602 is like a ring of sandpaper placedconcentrically around the aperture provided for the arbor of the anglegrinder. (In our prototypes, it is a ring of sandpaper glued onto thebacking plate, but some other durable material which digs into the backsurface of the sanding disk may be used instead--such as an insert of aknurled or deeply etched metal, or a portion of a plastic surfaceincorporating projections. The projections or rough surface may not benecessary. Spigots on a metal washer are one preferred formation of aroughened surface. A simple metal washer may suffice, if the disk istightened sufficiently against it. This concentric ring is intended togrip a sandpaper disk (such as FIG. 24) inside its tear-out hole zone,so that if the disk in use is exposed to too great a stress it will comefree of the backing plate which can no longer drive the disk. Anotheradvantage of this ring (as shown in the section 2600) is that the slightelevation of the gripping surface 2602 provides further air movementbetween the sanding disk and the backing plate 2603 during use, socooling the rear of the sanding disk.

In our opinion the grip pad and the drive pins are preferably not usedtogether; though this opinion depends on the relative effectiveness ofeach construction as it is implemented in a commercial embodiment.

FIGS. 27 to 30 show a contact sanding disk and a backing plate suitablefor use with such a contact disk. This type of disk is used particularlyfor finishing work on automobile bodies, for producing a smooth surfaceon or under painted layers. The user of this kind of disk is facedmainly with the problem of securing a long disk life before it getsclogged up, which requirement can also be expressed as the problem ofkeeping the disk and work surface cool during sanding. We havediscovered that a good vacuum can be created within the relatively thickbody of the backing plate during rotation, by making channels (see FIG.7; 706) which run substantially centrifugally, so that air is flung outfrom them and extracted from apertures (such as 2803 or 2905) passingthrough and near the centre of the contact adhesive disk. Theseapertures may also serve as locating or aligning holes. If the pins usedprojected right through the backing disk, it may be preferable to sealoff those holes with a flap of a resilient material, so that the effectsof the vacuum are concentrated on the abrasive surface. Preferably thechannels are exposed when the sanding disk is removed, so thataccumulated debris can be flushed out.

FIG. 27 simply shows the rear (operator's view) surface of an unmodifiedbacking plate having a nut 2701. Air extraction (vacuum) channels arenot shown. FIG. 28 shows a three-hole version 2800 of a contact sandingdisk with (a) vision/cooling apertures 2801 in three pairs of two, (b)indexing/alignment holes 2803, (c) fold lines 2805 about a cut 2804, and(d) vacuum and alignment apertures. Note that in this version the pairsof vision/cooling apertures 2801 are arranged to be not on radii of thedisk. The cuts 2804 allow the abrasive material to be deformed inwardsagainst corresponding depressions within the backing plate (see FIG. 23)and striker plates running along the line joining the apertures 2810 maybe installed. FIG. 29 shows another version of a contact sanding diskwith the 22 mm diameter vision/cooling apertures aligned along radii,(b) 8 mm diameter vacuum/alignment holes, and (c) fold lines.

FIGS. 30 to 33 show a four-sided sandpaper disk system. The disk3000--FIG. 30 has wing tips 3003 which help increase air flow betweenthe disk and the material being abraded, as well as reducing the impactof rim contact, four 16 mm diameter viewing holes 3001 which are theprimary source of ventilation, and a central tear-out hole zone 3002,inside an array of alignment holes 3004.

FIG. 31 shows at 3100 the four-sided sandpaper disk 3101 in positionupon (behind) a backing plate 3102. Note the alignment (any one of 4positions) of the viewing/ventilation holes in the sanding disk behindthe raked holes of the backing plate.

FIG. 32 shows the work surface side of a backing plate 3200 compatiblewith the sanding disk of FIG. 30. This plate has a grip pad 3203, fourcooling channels (3201), four structurally weakened breakout zones(holes 3202) in case some object projects through theviewing/ventilation apertures, and four index alignment apertures.

FIG. 33 shows a backing plate 3304 in section and a matching four-sidedsanding disk 3300, having four viewing/ventilation apertures withanti-snagging features 3303, thinned break-out zones 3301, and aconcentric weakened or tear-out zone inside the alignment holes. Thesanding disk also has wing tips 3302 (see above).

We estimate that a manufacture of four-sided sanding disk, wherematerial has been removed from the circumference, can involve a savingof at least 15% of the raw abrasive material over conventional circulardisks, because the cutting lines used for circular disks do not touchand there is a reasonably large amount of un-used material lying betweencircles. In contrast, a single cut can separate adjacent square-sideddisks. There is a little waste material where the corners of the squareshave been radiused; but this is relatively small.

FIGS. 34 to 37 show a three-sided sandpaper disk; similar to the abovefour-sided version. FIG. 34 shows a disk in position upon a suitablebacking plate 3400. One of three large viewing and ventilation holes,provided with an anti-snagging features, is at 3403. In case some objectcatches within this aperture during use, holes 3401 give the backingplate a weakened zone so that it can let the object through (We shouldsay that we find it almost impossible to male an object catch in theholes of a spinning disk; the most likely circumstances are when thedisk is spinning only very slowly).

FIG. 35 shows a backing plate 3500 compatible with the sanding disk 3600of FIG. 36, having a grip pad 3503, and index alignment holes 3502. FIG.36 shows a three-sided sandpaper disk 3600 with (a) wing tips (notlabelled), (b) ventilation/viewing holes 3601 fitted with anti-snaggingfeatures, (c) a concentric tear-out hole zone near the central aperture,at 3603, and (d) alignment holes 3602. FIG. 37 shows a backing plate insection (3705) and a matching three-sided sanding disk (3700), havingventilation holes 3702 with anti-snagging features, break-out zones 3701on the trailing side of the ventilation holes, and a concentric weakenedor tear-out zone 3703. Alignment holes are provided at 3704. The backingplate 3705 has a grip pad 3707--like a ring of sandpaper--intended togrip the sandpaper disk concentrically inside its tear-out hole zone.The area 3706 is provided with apertures for promoting air circulationfor cooling the working area during use. Wing tips are again providedand drawn, as at 3708.

Wing tips or deliberately formed vanes (either on the edge of thesanding disk, or made from the material of a backing plate) or evensimple deformations of the edge of a resilient backing plate may be usedto entrap air about the circumference of the sanding disk. These may beused in conjunction with an air containment "skirt" around the guard ofthe angle grinder and projecting towards the work surface, the skirtbeing made of a soft and preferably transparent resilient material (suchas polyurethane) and including a positioned gap placed so that dust isejected in one direction rather than in all directions. A dustcollecting device can then be installed so that a substantial proportionof the dust is retained. This type of guard is designed for use with thethick, resilient backing plates intended for use with contact sheets ofsandpaper and for use in applications such as automobile bodyworkfinishing; in manufacture or repair.

EXAMPLE

In this Example the advantages of the disks in which chord segments areremoved to produce an abrasive disk. In this Example, four disks arecompared for grinding performance. The first disk, (D), is a prior artdisk with a diameter of 11.4 cm (4.5 inches) with a central mountingaperture used in the typical prior art fashion with only the outerperiphery actually used for grinding. This was done by having the areaof contact on the workpiece overlap the perimeter. The second, (B) wasidentical to the D disk except that full contact was maintain with thefull workpiece by moving the location of engagement between the disk andthe workpiece to the same location used with the other disks. The thirddisk, (C), was an identical disk but modified to make it according tothe invention by being provided with three viewing apertures as shown inFIG. 24 (2400) of the drawings except for the omission of features 2401and 2402. The fourth disk, (A), was a disk similar to disk C except thatchord segments were removed to provide a disk as shown in FIG. 16 (1600)of the drawings. The backup plates were of 2.54 cm thick aluminum withshapes similar to the disk shapes as taught in the specification. Theabrasive surface was provided 50 grit fused alumina with phenolic makerand size coats.

The disks were evaluated using an Okuma ID/OD grinder used in anaxial-feed mode such that the workpiece was presented to the face of thedisk rather than an edge.

The workpiece used in each case was 1018 mild steel in the form of acylinder with an outside diameter of 12.7 cm (5 inches) and an insidediameter of 11.4 cm (4.5 inches). The end surface was presented to theabrasive disk. The abrasive disks were operated at 10,000 rpm and anin-feed rate of 0.5 mm/min was used the workpiece was rotated at 12 rpm.No coolant was used and the workpiece was centered on the portion of thedisk where the viewing holes are located in the embodiments according tothe invention. The disks were glued to the backup plate and this unitwas weighed before and after the testing.

To determine the reference point the workpiece was brought into contactwith the disk until the axial force reached 0.22 kg (1 pound). Grindingwas then continued from this reference point until the axial forcereached 1.98 kg (9 pounds), which was taken to correspond to the end ofthe useful life of the disk. Thus the time of grinding between thereference point and the end point was considered to be the useful lifeof the disk.

The results are represented graphically in FIGS. 37-41. From FIG. 38 itcan be seen that the rapid rise to a normal force of 9 pounds, which istaken to be the end point since at that point little metal removal isoccurring since most of the abrasive grit has been removed or worn out,occurs at about the same time for all all the round disks butsubstantially later for the disk A with the modified triangular shape.Indeed this disk lasted about twice as long as any other disk. This iscounterintuitive since more of the abrasive surface has been removed.

In FIG. 39, the power drawn by each of the disks was plotted as afunction of time. This showed the same pattern as FIG. 38 with the diskA drawing significantly less power throughout the period when all diskswere actually grinding. Thus disk D required less force and drew lesspower.

In FIG. 40, the friction coefficient variation with time is plotted forthe four disks. Her separation develops between the round disk with theobservation holes and the two prior art disks with a significantly lowercoefficient of friction being observed for the disk according to theinvention. However the lowest coefficient of all is observed with diskA.

FIG. 41 compares the amount of metal cut over time by the four disks.This shows that disks B, C, and D cut about the same amount of metalover the periods of the test but disk A cut about twice as much.

Thus the disks according to the invention cut at least as well as theprior art disks while affording the benefit of being able to view thearea being abraded as the abrading progresses rather than betweenabrading passes. This is very important for angle grinding particularly.Moreover this is obtained even though the amount of abrading surface isreduced by provision of the viewing holes. Most significantly however,when the abrading surface of the disk is reduced further by the removalof chord segments, (as in disk A), so as to give improved vision of thesurface of the workpiece right up to the edge of the abrading disk, thedisk cut more metal, at a lower power draw-down and over a longerperiod. This is quite unexpected and highly advantageous.

ADVANTAGES

Advantages of preferred forms of this invention include:

1. The user can see through apertures in the spinning tool to accuratelygrind a desired conformation, or shape;

2. However the apertures principally provide air turbulence across thework surface, assisting in debris removal and in cooling the sandingdisk and backing plate, so that the area being abraded remainsrelatively cool and under its melting point. One test showed a reductionof 114° F. difference on steel.

3. The sanding disk is worn more evenly, and lasts longer. The anglegrinder uses less power (as measured by driving it from alimited-capacity petrol generator).

4. There is less tendency for material to clog the abrasive surface.Dust is blown well away from the job.

5. The disk provides a finer and more even finish.

6. The invention is particularly useful in sheet metal work, where thelikeliness of the sheet metal becoming distorted due to heat generatedduring "cleaning-up" of welds or seams or the like by abrasion is low,thanks to the cooling effect of the apertures.

7. The adjustable guard assists in operator protection against arelatively "naked" spinning sanding disk.

8. The manufacturing process allows disks of any shape to be madewithout expensive dies.

9. More units can be made from the same amount of rawmaterial--typically over 15% more.

One might wonder whether a sanding disk with so much less actualabrasive material than a solid circular one represents value for money.In our experience the disks of this invention last significantly longerbefore replacement is needed. The cooler operation reduces clogging,keeps the work surface at a lower temperature, and reduces damage to thesanding disk. The wear patterns of our disks are superior, in that wearis more even, so that a disk reaches the end of its life much later. Thework is ground down more gradually and over a wider area, so that scoremarks and the like are less evident.

Finally, it will be appreciated that various alterations andmodifications may be made to the shape of the sanding disk and relatedequipment without departing from the scope of this invention as setforth.

What we claim is:
 1. An abrasive disk in the form of a circular diskhaving opposed first and second major surfaces wherein said first majorsurface is an abrasive bearing surface; a centrally located mountingaperture; and at least three identical, non-contacting, segments removedfrom the periphery of the disk at spaced locations around the disk. 2.An abrasive disk according to claim 1 in which from three to five chordsegments are removed from the disk.
 3. An abrasive disk according toclaim 1 in which viewing apertures are located in the disk between theremoved segments.
 4. An abrasive disk according to claim 3 in which theapertures have leading and trailing edges defined by the direction ofrotation of the disk and the trailing edges are deformed away from theabrasive bearing surface of the disk and towards the second majorsurface of the disk.
 5. An abrasive disk according to claim 4 in whichdeformation of the trailing edge is facilitated by the provision of aslit extending away from the trailing edge and adapted to permit thedisc to be deformed away from the abrasive bearing surface of the diskand towards the second major surface of the disk.
 6. An abrasive diskaccording to claim 1 in which from 3 to 9 symmetrically located viewingapertures are provided in the disk.
 7. An abrasive disk according toclaim 1 having an even number of viewing apertures located on evenlyspaced radii around the disk wherein the radial distance from the centerof the disk to the closest point for alternate apertures is greater thanthat of apertures located between said alternate apertures.
 8. Anabrasive disc according to claim 7 in which all the viewing aperturesare circular with identical dimensions.
 9. An abrasive disk according toclaim 7 in which the disk is provided with zones of weakness permittingrupture of the disk when subjected to excessive localised resistance torotation said zones of weakness being located between the viewingapertures and the periphery of the disk.
 10. An abrasive disc accordingto claim 1 in which the disk is provided with one or more peripheralfolds that are directed away from the abrasive bearing surface of thedisk.
 11. An abrasive disc according to claim 1 having, in the vicinityof and surrounding the mounting aperture, a weakened portion adapted torupture when the resistance to rotation of the disk when in use exceedsa predetermined amount.
 12. An abrasive disc according to claim 11wherein the weakened portion is provided by a ring of holes surroundingthe mounting aperture.
 13. An abrasive disk according to claim 1 whichis a rigid disk adapted to be fitted directly to the arbor of a grinderwithout a backing plate.
 14. An abrasive disk according to claim 13 inwhich the abrasive bearing surface is provided by a non-woven fibrousmat having abrasive particles adhered to at least some of the fibers.15. An abrasive disk according to claim 13 in which the abrasive bearingsurface is provided by a plurality of abrasive flap elements each havingan attachment edge and an opposed free edge, and in which the elementsare attached to the disk along their attachment edges and substantiallyeach free edge overlies the attached edge of an adjacent element so asto align the elements in overlapping relationship around the peripheryof the abrasive disk.
 16. An abrasive disk according to claim 15 inwhich the abrasive flap elements are located in groups spaced around theperiphery of the disk and between apertures in the disk.
 17. An abrasivedisk according to claim 1 in which the abrasive bearing surface isprovided by a fibrous mat having abrasive particles adhered to at leastsome of the fibers.
 18. An abrasive disk according to claim 1 which isprovided with air circulation holes located adjacent the mountingaperture.
 19. An abrasive disk according to claim 1 in which the disk isof metal and an abrasive material is metal bonded to the surfacethereof.
 20. An abrasive disk according to claim 1 in which the abrasivebearing surface is provided by a plurality of abrasive flap elementseach having an attachment edge and an opposed free edge, and in whichthe elements are attached to the disk along their attachment edges andsubstantially each free edge overlies the attached edge of an adjacentelement so as to align the elements in overlapping relationship aroundthe periphery of the abrasive disk.
 21. An abrasive disk according toclaim 20 in which the abrasive flap elements are located in groupsspaced around the periphery of the disk and between apertures in thedisk.
 22. An abrasive disk having a mounting aperture and opposed firstand second major surfaces wherein the first major surface is an abrasivebearing surface, said disk also having at least two non-concentricviewing apertures through the disk in which each aperture has leadingand trailing edges defined by the direction of rotation of the diskwhile in use, wherein the trailing edge is deformed away from theabrasive bearing surface of the disk and towards the opposed secondmajor surface of the disk.
 23. An abrasive disk according to claim 22 inwhich deformation of the trailing edge is facilitated by the provisionof a slit extending away from the trailing edge and adapted to permitthe disc to be deformed away from the abrasive bearing surface of thedisk and towards the opposed second major surface of the disk.
 24. Anabrasive disk according to claim 22 from 3 to 9 symmetrically locatedviewing apertures are provided in the disk.
 25. An abrasive diskaccording to claim 24 having an even number of apertures located onevenly spaced radii around the disk wherein the radial distance from thecenter of the disk to the closest point for alternate apertures isgreater than that for apertures located between said alternateapertures.
 26. An abrasive disc according to claim 22 in which all theviewing apertures are circular with identical dimensions.
 27. Anabrasive disc according to claim 22 having, in the vicinity of andsurrounding the mounting aperture, a weakened portion adapted to rupturewhen the resistance to rotation of the disk when in use exceeds apredetermined amount.
 28. An abrasive disc according to claim 27 whereinthe weakened portion is provided by a ring of holes surrounding themounting aperture.
 29. An abrasive disk according to claim 22 which is arigid disk adapted to be fitted directly to the arbor of a grinderwithout a backing plate.
 30. An abrasive disk according to claim 29 inwhich the abrasive bearing surface is provided by a plurality ofabrasive flap elements each having an attachment edge and an opposedfree edge, and in which the elements are attached to the disk alongtheir attachment edges and substantially each free edge overlies theattached edge of an adjacent element so as to align the elements inoverlapping relationship around the periphery of the abrasive disk. 31.An abrasive disk according to claim 30 in which the abrasive flapelements are located in groups spaced around the periphery of the diskand between apertures in the disk.
 32. An abrasive disk according toclaim 29 in which the abrasive bearing surface is provided by anon-woven fibrous mat having abrasive particles adhered to at least someof the fibers.
 33. An abrasive disk according to claim 29 in which theviewing apertures are raked with respect to the direction of rotation.34. An abrasive disk according to claim 22 in which the abrasive bearingsurface is provided by a plurality of abrasive flap elements each havingan attachment edge and an opposed free edge, and in which the elementsare attached to the disk along their attachment edges and substantiallyeach free edge overlies the attached edge of an adjacent element so asto align the elements in overlapping relationship around the peripheryof the abrasive disk.
 35. An abrasive disk according to claim 34 inwhich the abrasive flap elements are located in groups spaced around theperiphery of the disk and between apertures in the disk.
 36. An abrasivedisk according to claim 22 in which the abrasive bearing surface isprovided by a non-woven fibrous mat having abrasive particles adhered toat least some of the fibers.
 37. An abrasive disk according to claim 22in which the disk is provided with one or more peripheral folds that aredirected away from the abrasive surface.
 38. An abrasive disk accordingto claim 22 in which the portions of the disk between the viewingapertures and the periphery of the disk are provided with zones ofweakness permitting rupture of the disk when subjected to excessivelocalised resistance to rotation.
 39. An abrasive disk according toclaim 22 provided with air circulation holes located adjacent themounting aperture.
 40. An abrasive disk according to claim 22 whereinthe disk is of metal and an abrasive material is metal bonded to thefirst major surface thereof.